Implements for planting row crops, such as corn and soybeans, (planters) typically include row units laterally spaced along a frame, or toolbar. The row units generally include a seed channel opener that creates a channel or furrow in the soil for seed placement. Each row unit is mounted to the toolbar by means of a four-bar linkage or its equivalent which allows each row unit to move vertically to adjust to the contour of the soil independently of the other row units on the same toolbar. Some planters have springs in the four-bar linkage which work to transfer weight from the planter's frame to the row unit creating down force to help the seed channel opener penetrate the soil and to minimize row unit bounce in rough soil conditions. Insufficient down force can result in a seed furrow of inadequate depth or a seed furrow simply not formed, which in turn results in shallow seed placement or seed placement on the soil surface. However, too much down force could overly compact the seed bed or form the seed furrow too deep, which could negatively affect early plant development. Furthermore, excessive down force could accelerate wear on the row units' soil-engaging components. The springs can be adjusted to adjust the down force of the row unit. This adjustment usually is made by manually changing the position of the springs in the four-bar linkage.
In other planters, airbags are employed in the four-bar linkage which are similarly adapted to transfer weight from the planter's frame to the row unit creating down force to help the seed channel opener penetrate the soil and to minimize row unit bounce. In both of these conventional biasing means—springs and airbags—the system lacks accuracy and predictability. For instance, when the biasing means is an airbag, it can be difficult to precisely determine the volume of air in the airbag at a given time and, subsequently, determine needed supplemental down force.
It is desirable to be able to adjust down force on a row unit quickly and accurately so that a consistent seed depth is maintained. It is also desirable to be able to lift the row unit if its own weight is applying too much down force to the soil.